1. Field of the Invention
The present invention relates to cementitious compositions comprising Portland cement, coal combustion byproducts, expanded or extruded polystyrene, and, optionally, fiber. The invention further embraces lightweight structural units such as concrete masonry units (hereinafter "CMUs") produced from the cementitious composition of the present invention and a block machine used for preparing CMUs from the cementitious composition. The CMUs are used in the mining and construction industries.
2. Description of the Prior Art
It is well known in the mining industry to use CMUs to seal off areas in an underground mine which are not in use, and to direct the flow of air in areas which are in use, for the purpose of controlling ventilation in the mine. It is also well known to use structural units, such as CMUs, in the construction industry for the fabrication of masonry structures and the like.
Cementitious compositions used in the past to produce CMUs include combinations of Portland cement; coal combustion byproducts; sand, limestone and other aggregates; and water. These CMUs typically have a density ranging from 75 to 130 pounds per cubic foot, and have dimensions of approximately 8".times.8".times.16".
CMUs manufactured from these compositions are traditionally manufactured using one of two processes, typically depending on the water content of the mixture: (1) the block machine process, when the water content of the mixture is low (a mixture with low water content, i.e. from 0.01 to 5 gallons of water per cubic foot of dry mixture, is typically referred to as having a "low slump"), and (2) the "wet-pour" process, when the water content of the mixture is high (a mixture with high water content, i.e. from 5 to 10 gallons of water per cubic foot of dry mixture, is typically referred to as having a "high slump"). The manufacturing process utilizing the block machine, such as a Besser Block Machine, Model No. V312, includes feeding the cementitious composition into a mold, applying pressure and vibration while the composition is in the mold, extracting the resulting product from the mold and kilning and/or drying the product. Certain traditional CMUs must be manufactured using the "wet-pour" process due to the high slump of the cementitious composition used in the manufacture of such CMUs. The "wet-pour" process includes pouring the cementitious composition into a mold and, while still in the mold, kilning and/or drying the product. This latter process is more time consuming than the block machine process and requires the use of a number of molds during the manufacturing process. While a block machine requires an extensive start-up cost, ultimately the block machine process is less expensive and more efficient than the "wet-pour" process.
The CMUs produced from the traditional cementitious compositions described above are heavier than preferred for use in the mining industry. Preferably, the CMUs used in the mining industry are ultra-lightweight, i.e., having a density of from 25 to 45 pounds per cubic foot, and dimensions ranging in size from 6".times.16".times.24" to 8".times.16".times.24". A cementitious composition used in the past to produce such ultra-lightweight CMUs comprises Portland cement, coal combustion byproducts, soap bubbles and water. Due to the introduction of soap bubbles into the composition, the composition necessarily has a high water content, and therefore a high slump, to incorporate and maintain the soap bubbles. As a result, these ultra-lightweight CMUs must be manufactured using the "wet-pour" process described above, and therefore the resulting CMUs must dry for an extended period of time while still in the molds, which is more expensive and more time consuming than is the use of the block machine.
The ultra-lightweight CMUs produced from a cementitious composition comprising soap bubbles also tend, when subjected to certain environmental conditions (such as low humidity, vibration, foam density, and high heat), to lose their shape and to vary in size and density. Furthermore, the prior art ultra-lightweight CMUs tend to break and crack during their manufacture, shipment and/or installation.
Accordingly, there is a need in the art for a cementitious composition which can be used in a block machine for the manufacture of large and ultra-lightweight CMUs, which will maintain their shape (and thus size and density) and which are easily toolable with standard masonry tools, and which will not break or crack during their manufacture, shipment and/or installation.